On the ovogenesis and spermatogenesis of Sagitta bipunctata. 935 
the chromosomes and a stage like that shown in Figs. 24 and 25, 
where usually all the cells of a group contain nine distinct deeply- 
staining loops with a somewhat crenate or beaded outline. The 
number is easily counted in cross-section (Fig. 26). At this stage one 
or two large nucleoli are present, but the nuclear membrane is not 
easily made out. The chromatin loops which are oriented with the 
ends toward the center of the mass of cells, increase considerably in 
size and become more open and less stainable (Figs. 25 and 27). 
Then comes a stage in which the nuclear membrane is more evident 
and contains large granules, but the chromosomes are very indistinct, 
or in many cells cannot be seen at all (Fig. 28a); from such cells, 
the nine chromosomes again appear as somewhat shortened thick loops 
or V’s (Fig. 28b), that soon become more or less scattered in the 
cell, which has increased somewhat ‘in size. These loops shorten 
(Fig. 29) and straighten until they assume a dumb-bell form (Fig. 30), 
the form in which they appear in the aequatorial plate of the first 
spermatocyte division spindle (Fig. 31). Figs. 31—33 show different 
phases of the first spermatocyte division, Figs. 34—36 of the second. 
The two spindles are very much alike except in size, and both the 
cells and the chromosomes are smaller during the second division, but 
with some variation in size in different sections even from the same 
animal. The chromosomes in the equatorial plate in both cases have 
a dumb-bell form, and are so closely packed together that only occasion- 
ally is it possible to count them in a polar view of the equatorial 
plate as in Fig. 32, where nine are plainly seen. Fig. 33 shows a 
large nucleolus near one of the polar groups of chromosomes; often 
two are present and they may be found in any position with respect 
to the spindle. The centrosomes are very minute, and only in rare 
cases can be distinguished. The spindle fibres are very delicate, and 
no polar radiations can be seen. The fibres which connect the daughter 
groups of chromosomes are much more conspicuous (Figs. 33 and 36). 
In the same group of cells may often be found both spermato- 
cyte divisions and also cells like those in Fig. 37 where the chromo- 
somes from the last division, without an intervening resting stage, are 
beginning to form the sperm-head; Fig. 38 shows later stages of the 
same process and Fig. 39 still later stages, where the spermatids in- 
crease greatly in length, and gradually change their position, the cell 
walls having disappeared, until all the spermatids belonging to one 
group of cells are arranged in two bundles, as described by Lee. My 
material did not prove to be favorable for the study of the formation 
of the middle piece or the tail of the spermatozoon. It merely shows 
Zool. Jahrb. XVII. Abth. f. Morph. 16 
